What Does your Sunscreen's SPF Really Mean?
First things first. SPF stands for "sun protection factor." It is an indication of the protection against UVB (ultraviolet B) rays that cause sunburn. It is important to take into consideration the UVB rays when thinking about sun protection but also the UVA (ultraviolet A) rays. UVA rays cause skin cancer, are absorbed by the skin and are responsible for causing visible signs of aging and wrinkles.
When determining what SPF to use it really comes down to simple math. You want to take the length of time it takes you to burn without sunscreen by the SPF value to get your total sun exposure for the entire day. Say it usually takes 10 minutes for your skin to burn without any sunscreen and you use an SPF 30. You would take the length of time (10 minutes) by the SPF value (30) to get total length of time before you start to burn = 300 minutes. You would have 5 hours of sun exposure for the day before you start to burn. It is important to reapply every 2-3 hours because sunscreen can wear off with sweating, swimming, outdoor activities, etc. Even if your sunscreen claims to be sweat proof and provide all day protection, it can't if it's not fully applied for optimal protection. Also know that reapplying does not restart the clock. Your 300 minutes or 5 hours is it for the ENTIRE day!
Always apply sunscreen 20 minutes before you go outside and don't forget your ears!
I recommend using a physical sunscreen because the main ingredients of a physical sunscreen are Zinc Dioxide and Titanium Dioxide. They sit on the skin and don't cause reactions like many chemical sunscreens can. See below for my favorite sunscreens.